An acoustic resonator may act as a transducer that converts electrical signals into acoustic signals and/or vice versa. Examples of acoustic resonators include bulk acoustic wave (BAW) resonators, such as thin film bulk acoustic resonators (FBARs) and surface mounted resonators (SMRs), as well as surface acoustic wave (SAW) resonators. Acoustic resonators may used in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers, and other portable communications devices. For example, FBARs are commonly used to implement electrical filters, duplexers and voltage transformers in the above and other applications.
An acoustic resonator typically comprises a layer of piezoelectric material arranged between two conductive plates or electrodes, which may form a thin membrane. Two acoustic resonators can be coupled acoustically to form an electrical filter. When stimulated with a time-varying input signal from an input terminal of the electrodes, the piezoelectric material vibrates at a resonance frequency determined by physical properties of the acoustic resonator, such as its geometry and composition. This vibration produces a time-varying output signal at an output terminal of the electrodes.
One type of electrical filter implemented by acoustic resonators is a laterally coupled resonator filter (LCRF), which typically includes a ground plane, a piezoelectric layer and a set of interdigital top comb electrodes having interlaced comb-like fingers. Generally, an electrical signal is applied to one of the top comb electrodes of an LCRF, which excites Mason (or piston) mode under that electrode. Generally, Mason mode undergoes scattering at the electrode edges and produces spurious modes in the fingers and corresponding gaps between the fingers. The spurious modes in the gaps propagate to the fingers of the other top comb electrode, exciting motion. Voltage is generated by the excited motion, which is picked up as a transmitted signal.
There are a number of advantages to using an LCRF over other types of acoustic resonator filters, such as ladder filters formed of series and shunt resonators (e.g., FBARs) interconnected in a ladder-type structure. For example, the process of fabricating an LCRF is relatively simple, in that it essentially involves only top electrode patterning. Also, there is no need for mass-loading of various ones of the series and shunt resonators, and there may be a reduction in physical space required for the filter. However, LCRFs are generally difficult to design with regard to specific pass-bands. In comparison, a typical ladder filter requires only one-dimensional Mason model simulations, whereas an LCRF requires two-dimension or even three-dimensional finite element method (FEM) model simulations. Also, spurious pass-bands may be present in various spectral regions. The embodiments described herein address these and other issues, including suppression of spurious pass-bands, through apodization of LCRF geometry.